Remove unparsable jobs from the in-flight list.

If a job description could not be parsed, e.g. because the JSON contains
errors or could not be mapped to a particular class, it should be removed
from the in-flight list.

Not removing such jobs from the in-flight list can lead to infinite
requeueing.
Merge pull request #50 from danieldk/jackson-exception

Remove unparsable jobs from the in-flight list
Make pop() protected

From https://github.com/tjhruska/jesque/commit/88c9b0bbe9b2a2e0cecb93331fffcd43f0f82cf2